This invention describes a novel and useful process for oxidizing 1,2-diphenyl-4-[2-(phenylthio)ethyl]-3,5-pyrazolidinedione with peroxymonosulfuric acid to obtain 1,2-diphenyl-4-[2-(phenylsulfinyl)ethyl]-3,5-pyrazolidinedione. The latter compound is useful as an uricosuric agent.
An oxidation of 1,2-diphenyl-4-[2-(phenylthio)ethyl]-3,5-pyrazolidinedione using hydrogen peroxide is described by R. Pfister and F. Hafliger, Helv. Chim. Acta, 44, 232 (1961). Although the latter report describes the preparation of 1,2-diphenyl-4-[2-(phenylsulfinyl)ethyl]-3,5-pyrazolidinedione in reasonable yield, hydrogen peroxide is not suitable as an oxidizing agent when a number of large scale reactions have to be performed. In large scale preparations, use of hydrogen peroxide gives unpredictable yields of the sulfinyl compound, sometimes unacceptably low. Furthermore, with hydrogen peroxide, a major side reaction is the concurrent oxidation of the sulfoxide to the corresponding sulfone which is very difficult to remove. Several other oxidizing agents, for example, N-bromosuccinimide, sodium metaperiodate, chromic acid-pyridine, chromic acid-glacial acetic acid, perbenzoic acid and manganese dioxide, were tried. Each of these agents failed to produce the desired sulfinyl compound. Although m-chloroperbenzoic acid, when used as the oxidizing agent, gave the sulfinyl compound in reasonable yield, m-chloroperbenzoic acid is not acceptable because of its high cost.
Surprisingly, use of peroxymonosulfuric acid, also known as Caro's acid, as the oxidizing agent gave consistent and acceptable yields (about 60%) of the desired sulfinyl compound and very small amounts of the undesired sulfone (less than 5%). Furthermore, Caro's acid is a very inexpensive oxidizing agent.
The effectiveness of Caro's acid as an oxidizing agent is unexpected. More explicitly, diphenyl sulfide has been oxidized by Caro's acid to give diphenyl sulfoxide in 39% yield by I. P. Gragerov and A. F. Levit, Zh. Obshch. Khim., 33, 543 (1963). Diphenyl sulfide is a typical aromatic sulfide. The starting material used in this invention also is an aromatic sulfide and would be expected to behave as a typical aromatic sulfide; however, it does not. Most aromatic sulfides can be oxidized in 70% dioxane in water to the corresponding sulfoxide with an equivalent amount of N-bromosuccinimide. On the other hand, the reaction of N-bromosuccinimide with alkyl sulfides gives no sulfoxides, W. Tagaki et al., Chemistry and Industry (London), September 19, 1964, p 1624. The starting sulfide used in this invention failed to be oxidized to the sulfinyl compound under similar conditions using N-bromosuccinimide. Thus, the lack of reactivity of 1,2-diphenyl-4-[2-(phenylthio)ethyl]-3,5-pyrazolidinedione, when compared to typical aromatic sulfides, would predict that oxidation of the starting sulfide used in this invention with Caro's acid should give very low yields of the corresponding sulfinyl compound. However, the reverse was discovered, Caro's acid is an effective and safe oxidizing agent for selectivity converting 1,2-diphenyl-4-[2-(phenylthio)ethyl]-3,5-pyrazolidinedione to the corresponding sulfinyl compound.